Radio navigation system



Feb. M, E950 G. PAINE E'rAI. 2,497,513

RADIO NAVIGATION SYSTEM 5a\ B j l EcEII/EIE 27 I coMPoNENT II- TEANSMITTEIE [Fl /58 53\ I j? I TEANSMITTEIE 6 54` I l f "D" j i TIME DISPLA ?E- TSMITTEIE I Y FIG. I

AUToMATIc ZZ7 o I. I Assghglm vJIlIo\ SIGNAL EFC'E I OKC sYNcHRoNIzATIow I 'MAINTEIIAIIcE COMPONENT zoIcc.J I \73 l I 76 I I PuLsE- couNTINe VARIABLE I CHAIN vELocITY I I -SIGNAL PHASE I i |Ig\ /(l v `95 SHIFTE I I PEDESTAL Z5 '/II-f IIe GENERATOR ,Swggp- Y' I ASSEMBLY GENERATOR .f I I I SYIIIcHaoNIzm-ION f AND I TIME DISPLAY GEORGE R. PAINE FI@ 5 WILLIAM CI. PLATT INVENTORS ATTORNEY Feb. 14, 1950, Y G. R. PAINE HALl 49.75%

RADIO NAVIGATION SYSTEM Filed June 10, 1948 l5 Sheets-Sheet 2 FIG. 3A FiG. 3B F|G.3C

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w10 NAVIGATION sIIs'I'ExII Filed June 10, 1948 15 Sheets-Sheet l5 B --\/\/\/\/L VQ/v /Ga \95 I\ y +Ioo i FIG. 9

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ATTORNEY Feb. 14, E950 G. R. PAlNE ETAL 2,497,513

. RADIO NAVIGATION sysm Filed June 1o, 194s 15 sheets-sheet s +soo Pian GEORGERPAINE WILLIAM A@PLA-:#1:

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RADIO NAVIGATION sysma Filed June 10, 1948 15 Sheelbs-Sheet 9 woRNEY Feb. 14, 1950 G. R. PAINE ErAL RADIO NAVIGATION SYSTEM 15 Sheets-Sheet 10 Filed June 10, 1948 Nw mwN www ATTORNEY Feb. 14, i956 G. R. PAlNE Erm. 2,497,513

RADIO NAVIGATION svsma Filed June 10. 1948 15 Sheets-Sheet 11 'S 9.9 rf MAF-4| 650265 2. PAINE WILL/AM C. PLAT' INVENTORS www ATTORNEY Feb. 14, 1950 G. R. PAINE ErAL RADIO NAVIGATION SYSTEM 15 Sheets-Sheet 12 Filed June 10, 1948 ATTORNEY Feb. 14, 1950 G. R. PAINE ETAL 2,497,513

RADIO NAVIGATION SYSTEM Filed June 10, 1948 15 Shee'tzs-Sheet 13 PauimwN/w ff\ I m 1 /n L- /500-50/1/ ,us JP/00,11: -L- /00//5 -l i 77ME, "A" TRACE ATTORNEY Feb. 14, 1950 G. R. PAlNE Erm. 2,497,513

RAnIo NAVIGATION SYSTEM Filed June Io, 1.948 I 1S sheets-sheet 14 Feb. 14, 1950 G. R. PAINE TAL I2,497,513

RADIO NAVIGATION SYSTEM Filed June l0, 1948 15 Sheets-Sheet 15 BYW/ ATTORNEYl Pafeefed Feb. 14, 195o amro NAVIGATION srs'rmu George R. Paine and' William C. Platt, Pleasantville, N. Y., assignors to General Precision Laboratory Incorporated, a corporation of New York Application June l0. 1948, Serial No. 32,096

15 Claims. (Cl. 343-103) l This invention relates to radio navigation systems depending on the measurement of the time of arrival of two or more radio signals sent synchronously from known points.

Navigation systems of this type, such as that commonly called Loran, employ pairs of ground stations, each pair transmitting pulses of radio energy with a precisely xed time relation between the two pulse transmissions. A receiving station on a ship or airplane, by measuring the difference in time of reception of the pulses from a transmitting pair, learns that it is somewhere on a line on the earths surface where the signals would be received with such a time difference. By tuning to a second transmitting pair and again measuring the signal time difference, position on a secondv line is determined; the intersection of the two lines of position represents the vehicles position. Special-maps carrying time difference lines and indicating locations of, and data for, many pairs of transmitting stations cover some 25% of the earths surface and are in general use.

The speed of radio signals in space being that of light, the time intervals which have to be measured by the receiving station are small; in fact, they must be measured with an accuracy of about one millionth of a second (es). Such a receiving station therefore contains an electronic clock or oscillator capable of great accuracy, from which are generated several electrical frequencies used to measure the small time differences of the incoming pairs of signals.

The actual measurement is done visually, therefore, receiving stations include a cathode ray tube upon the screen of which are displayed the received signals, or pips, together with locally generated signals for synchronization of the receiver with the received signals and for measurement of the difference in time of reception of the signals. There may also be displayed on the screen locally-generated luminous linear scales marked in microseconds for use in measurement by the operator of the signal time differences or, instead, measurement may be made in a separate unit and not on the cathode ray tube screen.

Heretofore it has been necessary to adjust by hand the position of one of the received signal pips to a particular point on the scale representing zero time and to maintain it there by continuous manual readjustment, so that the distance in scale divisions and microseconds to the second received signal pip can be measured.

To accomplish this manual adjustment a switch has customarily been provided for adjusting the position of one of the received signal pips to a particular scale point representing zero time; the operation of this switch causes the pips to drift or move along the scale toward the right or left. When the pip arrives at the point judged by the operator to be its required zero position, he restores the switch. This operation has been found in practice to have two disadvantages: the speed of drift may be too slow and the operation therefore very time-consuming, taking minutes if the distance .to be drifted is great, and on the other hand unless the speed of drift is exceedingly slow the pip can only with dimculty or by chance be stopped at exactly the required point.

`This invention substitutes variable velocity adjustment of zero pip position for the previous type which may be termed displacement adjustment. It eliminates both disadvantages of the previously-used method and in addition employs automatic zero velocity restoration. The invention is especially effective'when used in conjunction with automatic signal synchronization maintenance, to be described later.

In the measurement of time differences by received signals three or four scale or trace magnitudes on the screen are customarily used consecutively to permit flrst rough synchronization and perhaps measurement, then repetition with more and more accuracy. At all times it has heretofore been necessary manually to maintain one of the signal pips in the aforementioned position representing zero time to compensate for fortuitous oscillator drift, by momentary operation of the above-mentioned switch or by change of the basic oscillator frequency. Likewise measurement of time diierential involves some guesswork, as the signal pip will have slanting sides and may be of an irregularshape, constantly changing as radio space transmission conditions change from instant to instant. Other interference, classed as video noise, may make the signal jump about, and unless the operator is constantly alert, this zero-set lpip may drift or jump out of setting unnoticed. There thus has been an error of observation involved in this `zero setting maintenance, which has added to any error of observation that may have occurred in the actual scale reading.

This invention eliminates the necessity for continuously readjusting one of the received pips to a point representing zero on the screen, this continuous readjustment being accomplished automatically, without any attention by the operator except under conditions of extraordinary noise interference, and the adjustment is maintained far more reliably and stably than is possible by hand.

An object of the present invention is to provide a novel and improved combination of means for long range navigation using radio signals received from pairs of iixed transmitting stations.

Another object of the present invention is to provide an improved method of and means for signal synchronization in such navigation means.

A further object of this invention is to provide an improved method of and means for automatic signal synchronization maintenance in such navigation means.

Other objects, features and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawing, in which:

Figure 1 is a functional chart of the principal components of a long range navigation system.

Figures 2, 3A, 3B, 3C and 4 illustrate various cathode ray screen aspects.

A Figure 5 is a functional chart of the timer oi' a marine Loran receiver.

Figures 6, 7 and 12 are functional charts of components of the timer.

Figures 8, 9, 10, 11, 13, 14 and 16 show schematically the wiring of component circuits of the timer.

Figures 15A, 15B, 15C and 15D show graphically time-potential relationships in the iine delay unit.

Figure llE shows schematically the mechanical arrangement of the counter.

Figure 15F illustrates the physical relations of the differential and counter.

Figure 1'7 is a functional chart of the automatic signal synchronization maintenance unit.

Figure 18 illustrates the operation of the automatic signal synchronization maintenance unit.

Figures 19 and x20 show schematically the wiring of the automatic signal synchronization maintenance unit and the variable velocity signal phase shifter.

Figures 21.22 and 23 illustrate the construction of the variable velocity signal phase shifter.

In Figure 1, four transmitters, 5i, 52, 53 and 54 form two pairs, the two stations of each pair transmitting on the same radio frequency and at the same pulse repetition rate, and maintaining a precisely designed and accurately maintained synchronization between the pulse transmission times from the two stations. Specifically let it be assumed for purposes of illustration that transmitters 5|. termed A or master station, and 52, termedl"B or slave station, transmit on 1950 kilocycles radio frequency at a pulse repetition rate of. 25 per second, Where kilocycles here and hereafter are taken to mean kilocycles per second.ff A receiver incorporating this invention is indicated by the dashed enclosure 56 and may be called for convenience a Marine Loran receivei. Its larger components, excluding power supplies and other usual parts, include a receiver component 51, timer 58, synchronization display 59 and perhaps a separate time display 6I, which may' however be combined with the synchronization display.

The receiver component may be similar to that described in Electronics Magazine for December, 1945, vol. 18, No. 12, page 113. In general this component receives through the receiving ante'rina 62 radio frequencies which have been pulsed from the transmitters (frequencies being understood to mean here and throughout.

electrical quantities of said frequencies). The receiver distinguishes between different radio frequencies, selects the desired frequency for which it is set, demodulates it and delivers the signals as direct current pulses (video signals) to timer 58 and to synchronization display 59.

Timer 58 receives the direct current pulses (video signals) from the receiver component and also generates six electrical pulse repetition frequencies for use in measuring the time intervals between the pulses transmitted by A transmitter 5| and "B" transmitter 52, called hereafter A pulses and B pulses. The six frequencies are related to one another geometrically and can be synchronized with the received pulses both as to rate and phase. 'I'he output of timer 58 consists o! various pulses and other wave forms impressed 0n synchronization display 59, and may also include mechanical shaft rotations transmitted to time display BI if the latter is separate from the synchronization display.

Synchronization display 59 consists of a cathode ray tube and associated equipment. A suitable tube is described in Radar System Engineering by Ridenour, 1st edition, page 499. The tube contains two pairs of electrostatic deecting plates, the horizontal" pair giving horizontal deflections to the cathode ray, and the vertical pair giving vertical deflections. It also contains the usual cathode, control grid, second control grid and fluorescent screen. Tube elements receive pulses and potentials from timer 58 and video signals from receiver component 5l, so that the ray traces one or two luminous horizontal lines with vertical humps or pips representing received signals, and pedestals and vertical lines which mark subdivisions of the horizontal lines and therefore represent various time subdivisions in microseconds.

Transmitted signals such as Loran signals are not only characterized by specific radio frequencies and by a basic pulse repetition rate, but in order further to distinguish between pairs of transmitters an additional small variation in pulse repetition rate between pairs of stations is employed. Eight variations are in use, known by digits zero to 7 inclusive, called station numbers, and differ successively in periods by microseconds. Three basic pulse repetition rates are in use, being 33%, 25 and 20 pulses per second and characterized by letters H, L and S respectively. Representing the basic pulse repetition period by P and the station number by N, the pulse repetition period is P-100 N microseconds.

For instance, using basic pulse repetition rate L and station number 5, the pulse repetition period is 40,000100 5=39,500 microseconds In one embodiment of this invention there is associated with synchronization display 59 a sixposition function switch. The first three positions are for use in' the normal functioning of the Marine Loran receiver, in which the securing of a "ilx is divided into three corresponding steps. The remaining three positions are for installation and maintenance. purposes. Other switches are for selecting the proper radio frequency channel, the basic synchronization rate, and the station number.

The cathode ray tube in all cases displays two horizontal sweeps from left to right, and the number of sweeps per second is always twice perimposed in function switch position 3.

It is necessary for the cathode ray to return or retrace to the observers left after tracing a sweep. This is accomplished very rapidly and with the ray intensity reduced, so that its retrace path is not visible to the eye.

In operation, lack of synchronization between the pulse repetition rate of the transmitting station pair and the sweep rate of the synchronization display results in movement of video pips along the sweep trace, while at exact synchronization the pips are stationary. The operations of synchronization and of correct placement consist, in function switch position l, of changing the sweep .rate until the A pulse of the desired video signal is stationary near the left of the upper trace; the B pulse will then be further toward the right on the B trace. The appearance of the synchronization display screen at this juncture is illustrated in Fig. 2. A pedestal or rectangular raised portion 63 is evident near` the left end of the A trace, and another, 64 on the B trace. Each pedestal is 1000 as long. The A pulse is made approximately stationary by adjustment of the oscillation frequency, then is moved tothe A pedestal by momentary manipulation of the variable speed drift adjustment. Next, the B pedestal is moved by adjustment of the coarse and fine delay knobs until it is under the B pulse. The function switch is now turned to position 2. which does not change the sweep repetition rate but enlarges the scale or size of the picture until the 1000 ps pedestals cover the entire sweep lengths. Appearance of the A and B pulses may now be as illustrated in Fig. 3A. By further adjustment of the coarse and line delay knobs the B pulse is now set directly under the A pulse as shown in Fig. 3B and both are moved simultaneously by momentary manipulation of the variable speed drift adjustment until the A pulse is caught and held in its exact position near the left end of the A sweep by automatic operation of the synchronization maintenance. To

facilitate this operation a 100 ,is pedestal v66? called B gate is evident near the left end of the A trace. This pedestal is the area to which the A pulse should be moved manually, and the left edge of this pedestal is the point to which the center of the A pulse is automatically drawn and locked as soon as it enters the gate. The result is shown in Fig. 3C. A sweep collapse key permits superposition of A and B sweeps in function 2 if desired. Changing to function switch position 3 causes A and B traces to coincide and enlarges the scale so that the first 200 ,is of the 1000 as length occupies the entire screen. Appearance is as in Fig. 4 where 61 represents the 'A pip and 68 the B pip. Adjustments of the position of the B pip and of the magnitude of either the A or B pip are now made so that its leading edge 69 coincides with leading edge 1| of the A pipe, the latter still being locked automatically in-the correct location on the trace. Synchronization of the receiver timer sweep frequency This may be done either by superposingupon the cathode ray tube screen sets of luminous linear scales marked in microseconds, and comparing the video pips therewith as described in Electronics Magazine, supra, page 111, or by use of a separate time display, or by any other suitable method of measurement to a time scale o! the time intervaljbetween reception of an A pulse and the companion B pulse.

In Fig. l5 the timer 58 of Fig. 1 is depicted in greater detail, and synchronization display 59 and time display 6|' are combined in one rectangle 12. Video signals from receiver component 51. Fig. 1 enter synchronization maintenance component 13, Fig. 5, through lead 221 and also enter synchronization and time display.

component 12. In synchronization maintenance component 13 these signals, if they occur in time within 100 as of their appropriate position in the sweep cycle, through lead force local oscillator 14 to change its rate of oscillation, thereby momentarily changing the rate of repetition of the sweep cycle to bring said signals into exactly proper position in the sweep cycle and to hold them there. This circuit will be described in greater detail later. With it is associated variable velocity signal phase shifter circuit H6, the function of which is to bring the signals initially near enough toward synchronization for the synchronization maintenance circuit' 13 to seize them, bring them to exact synchronism, and maintain them so.

Oscillator assembly 14 is shown in greater detail in Fig. 6. It includes a primary standard reference quartz crystal oscillator and a 20 kc. oscillator, the output of which is converted to very accurate ,is markers on which are based the several marker outputs of the pulse-counting I be slightly changed by the automatic signal synchronization maintenance circuit through lead I 40. Oscillator 4 feeds the control grid of pentode buil'er amplier i8, which in turn through a cathode follower connection feeds the suppressor grid of pentode mixer i1. The latter may be connected in any usual manner, as for instance as shown in an instruction book published by the General Radio Co., for its beat frequency oscillator model 713B. Mixer I1 also receives through its control grid a standard 20 kc. frequency generated as follows:

Stable 100 kc. crystal oscillator I may be of any conventional type, such as that described in Frequency Control With Quartz Crystals, published by Bliley Electric Co., 1940 edition, on page 24. It supplies 100 kc. energy to 5/1 sine wave divider 2, the design of which was described by Ernst Horrman in Proc. I. R. E. for October, 1946 under title, The inductance capacitance oscillator as a frequency divider. The output of divider 2, being accurately of 20 kc. frequency, is as before stated, fed to the control grid of mixer i1.

Mixer l1 secures its B power through resistor |43 shunted by a 1)ip-Watt neon lamp |4| and protective resistor |42. When the frequency supplied by the .standard 20 kc. frequency to the control grid and that supplied by oscillator e to the suppressor grid are the same, the B current will be constant and the neon lamp will not glow. But if the two 20 kc. supplies are somewhat out of synchronism the two will be in phase at one instant and out of phase at another instant, thus causing the neon lamp to glow at the beat rate. By adjustment of the oscillating circuit of oscillator 4 its output may be exactly synchronized with that of crystal oscillator l.

Buffer I8 thus will have consequent upon the above adjustment an accurate 20 kc. output. It supplies the fundamental timing referencefrequency to the pulse-counting chain, the outputs of which control and time all operations in synchronization and time display 12, Fig. 5. Buffer I8 also supplies 20 kc. through lead |32 to the phase-shifting bridge 30. (Fig. 12) to be described later.

Returning to Fig. 5, oscillator assembly 14 supplies 20 kc. alternating current to pulsecounting chain 16. This chain reduces the 20,000 cycle input frequency to lower output frequencies, and transforms the sinusoidal input to pulse or square wave outputs. It does this in five stages, supplying intermediate pulse frequencies from two of the intermediate stages. The next-to-last stage and the last stage each supply any one of three frequencies as' controlled by the position of the basic repetition rate switch.

The stages of the pulse-counting chain are pu1se-counters rather than frequency dividers. They are so constructed as to be responsive to individual pulses of potential rather than to be frequency-responsive. Each stage is designed to absorb and to store within itself a prescribed number of pulses and to re on a particular pulse, passing its output to the next stage and resetting itself. The frequency, intensity and wave shape of actuating pulses are of little importance within considerable limits, operation of stages being effected by reception of the designed number of pulses.

The five stages are shown in Fig. '7. At three places in the chain, trigger or pip-generating circuits are employed to produce short, sharp pulses of voltage for exciting the following stage and for timing purposes elsewhere within the timer. The first of these circuits, |9, is for the purpose of producing from the 20 kc. output or buffer I8 a short, sharp negative pulse, or pip as input to pulse-counter 20, the first stage of the chain, and also for supplying through lead |08 pips at intervals of 50as to injector 33, to be described later. It utilizes the well-known blocking oscillator circuit, described by Ridenour, supra, page 502. The second and third pip-generating circuits, 24 and 29, employ gas tubes. A schematic diagram of the circuit of pip generator 24 is shown in Fig. 8. Gas tube 81 receives its initiating pulse in the form of a strong negative wave with a very steep front' from pulse-counter 22, through coupling condenser 11. This momentarily makes cathode 18 highly negative with respect to grid 19, ring the tube. This produces a sudden but quickly dissipated negative pulse at lead 8| in the plate circuit, which pulse is utilized to initiate pedestal delay circuit 36 and trigger selector 38, both to be described later. Simultaneously, the voltage drop in cathode resistor 82 is transmitted as a positive 500 s terminating pip through lead 86 to coarse timing circuit 39, to be described later. Condenser 83 effectively short-circuits resistor 84 for pulses, intensifying the positive peak of the pulse transmitted through lead 86. The tube in ring draws upon energy stored in distributed capacitances. When this energy has been substantially used up, the discharge is extinguished, resistor being large enough that the current through it will not support the discharge.

The same negative impulse from 5-pulse counter 22 which res pip generator 24 also through lead. |09 fires 5pulse counter 25, Fig. 7, the next link in the counting chain.

Pip-generator 29 employs the gas tube circuit shown in Fig. 9. Normally the cathode is at +100 volts while the control grid is at ground potential. A strong negative pulse from pulsecounter 21 through condenser 98 drives the cathode potential downward to the point where the tube res. The resulting negative pulse from plate junction 89 goes through lead 9| to 2-pulse counter I2, Fig. '1, and through lead 95 to sweep generator I4, to be described later. The output also goes through lead 93 to pedestal delay 36, to be described later. Condenser 92 increases the width and energy of the pulse and retards its decay. When the energy stored in condenser 92 has been substantially used up, the discharge stops as the resistance of resistor is too high to support discharge.

Pulse-counters 20, 22, 25 and 21 are in general similar to each other. Each employs a duo-diode and a gas tube as shown in Fig. 10. This circuit has the advantages that Within reasonable limits its accuracy of counting is independent of the frequency of the incoming triggers, and is practically independent of uctuations in voltages supplied to tube plates and cathodes if the incoming trigger is supplied by a similar circuit. When quiescent, the anode 94 of tube 20A, anode 96 oi tube 20B and cathode 98 of tube 91 are all at approximately volts potential. Control grid 99 is at ground potential and no tube currents now. A strong negative pulse charging coupling condenser |0| goes through tube 20B to condenser |02, the charge of condenser |0| thereby being shared with condenser |02 and the potential of cathode 98 being brought to that of the initial pulse times the ratio of condenser |0| capacitance to the sum of the two capacitances, or

Where Ek is the potential of cathode 98 E: is the potential of the strong negative pulse which charges condenser |0| C101 is the capacitance of condenser |0| C102 is the capacitance of condenser |02 This potential assumed by cathode 98 is therefore definitely determined by the capacities of condensers |0| and |02 and by the input pulse potential, and the ring of the tube is determined -by the cathode and grid potentials. Ifthis pulsecounting stage is adjusted to count two pulses, the rst pulse will not make cathode 98 negative enough to re the tube. The succeeding input pulse will again charge condenser |0| which will again share its charge with condenser |02. In doing so, however, the voltage through which the cathode |03 of tube 20B must fall before it reaches the voltage of cathode 98 of tube 91 must be subtracted from Et, the trigger voltage. This results in decreases in the amplitudes of successive negative charges added to cathode 98. However, if adjusted as premised, gas tube 91 res on the second pulse, suddenly increasing its cathode potential to approximately +100 volts inthe followl 9 ing manner. Condenser |04 before ming is charged to +300 volts.` Upon firing it must instantly share its charge with condenser |02. But

since the size of the latter is made approximately' tance C, resulting from the imposition upon the condenser of a quantity of electricity Q.

It is obvious that anode 94 cannot ever retain j a potential substantially greater than +100 volts, for any potential materially in excess'of 100 volts of either cathode 98 or anode 94 would result in a discharge through tube A to a level of api proximately +100 volts, which insures that the quiescent potential from which cathode 98 starts to count shall never be more than approximately 100 volts. Resistor |06 limits the peak current n flow through gas tube 91 to preserve it from destruction by momentary high-current condenser discharges. Resistor |01 has a resistance so high that the current which may be drawn through it by the red gas tube will not support discharge.

Consequently, when the ilred tube has substanis.

tially exhausted the energy stored in condenser |04 its discharge ceases.

Pulse-counting stage 21 differs from stages 20,

22 and 25 as just described only in that it may be switched so that the interval between output pulses is 15,000, 20,000 or 25,000 ps. This is done by switching a grid bias potentiometer so that the stage will fire on the 6th, 8th or 10th pulse respectively. This stage. in addition to actuating trigger 29, also sends pulses through lead H3 to injector 33 to be described later.

Two-pulse counter l2 in Fig. 7 consists of a multivibrator of the type commonly called scaleof-two, which operates to produce a half-cycle square Wave when initiated by a negative pulse, and terminates said half-cycle and also commences the opposite-sense half-cycle when actuated by a succeeding negative pulse, maintaining the duration of said opposite-sense half-cycle until the next initiating negative pulse, and so on indeiinitely. Such a circuit is described by Ridenour, supra, page 499.

This last pulse-counter stage I2 is followed by two cathode follower ampliers Il and i3, to avoid excessive loading of stage I3. These both amplify the square-wave output of counter I2, but in opposite phase. Such a cathode follower amplier is described by Ridenour, supra. page 494.

The outputs of stage |2 and of cathode follower ampliiiers and I3 are 33%, 25 and 20 cycles,

or half the frequency of the preceding 6, 8, 10

pulse-counting stage, and twice-the period, or 30,000; 40,000 and 50,000 as. These are the basic periods of the timer. During each basic period the cathode ray makes two visible sweeps, called the A sweep and B sweep. During the half cycle in which A sweep occurs, ampliiier output is..

zero volts, and during the B sweep it is nearly +300 volts. Amplifier I3 output is the reverse of this. During A sweep it is nearly +300 volts and during B sweep it is zero volts. The output of amplifier goes through lead lll Vto the vertical plates of the cathode ray tube for trace separation, through lead I2 to pedestal generator trigger circuit 44 to be discharged later, through lead ||3 to trigger selector circuit 30 to be described later and through lead ||4 to the autol0 matic signal synchronization maintenance circuit 13 to be described later.

Injector 33 is an auxiliary of the counting chain. Its function is to reduce the sweep period in 50 is steps in accordance with the adjustment of the 8point station selector switch. It also blanks out the trace at the start of the slow sweeps. In Fig. 1l, the injector circuit is shown schematically. Two wafers ,of switch S2 are shown, with points marked in station numbers" from 0 to 7. 'I'he sweep period is controlled by their setting and i-s the basic period minus 50 N as where N is the stationnumber. As the pulsecountlng chain operates, 50 as negative pulses .from blocking oscillator |9 enter the injector through lead |08. and basic sweep period negative pulses of 15,000, 20,000 or 25,000 ,us enter the injector from pul-se counter 21 through lead ||9.

(see Fig. 1). Each latter pulse reduces thel potential of cathode |2| but not enough to cause tube 33 to fire. However, in combination with a negative pulse 50 as later through lead |08 tube 33 does fire, producing a strong negative pulse through lead |22 to switches S2-2 and S2-3 and condensers |23, |24, |26 and |21,.and through a combination of them to the cathodes of either or both of the as and 500 as stages `20 and 22 via diodes 35A and 35B and leads |28 and |29 respectively. The action of the pulse on pulse-counters 20 and 22 is additive as in regular counter action, and reduces by one the number of pulses from 50 as oscillator I9. necessary to cause pulse-counter 20 to fire when the station number is odd, and reduces by 1, 2 or 3 the number of pulses from stage 20 necessary to cause counter 22 to iire when the station number is 2 or 3, 4 or 5, 6 or 7, respectively. A negative pulse through lead |3| to the control grid of the cathode ray tube renders this tube inoperative for about 1000 as at the start of each A and B slow trace.

In Fig. 5 pedestal generator assembly ||8 is shown to be actuated by pulse-counting chain 18 and as actuating synchronization and time display 12. The function of the pedestal generator assembly is to supply the cathode ray tube vertical plates with increased potential at denite times and for denite periods so as to produce raised platforms or pedestals over part or all of the screen trace. The pedestals on the A trace are different from those on the B trace and are different on the slow, intermediate, and fast sweeps. The pedestal generator itself, consists of only one tube but with it are associated several other circuits. All these together are referred to as pedestal generator assembly ||8 and yare shown in greater detail in Fig. l2.

The pedestal generator assembly, Fig. 12, consists principally of four time-interval generating circuits; the pedestal delay generator, the ad- 'justable coarse delay circuit, the adjustable ne delay circuit and the pedestal generator. A phase-shifting circuit is auxiliary to the ne delay circuit. Three mechanical connections to a separate time display when used are indicated in Fig. 12 by da-shed lines |00, |05 and H5; these connections will be more fully described later in connection with the description of the time display.

Phase shifting bridge 30, Fig. 12, is shown schematically in Fig. 13. It receives 20 kc. energy through lead |32, Figs. 6 and 13, from the cathode of buffer-cathode follower amplifier I8. This energy in transformer primary |33 appears at the two end terminals ofsecondary |34 as two 

